1. Field of the Invention
The present invention relates to cold-rolled steel sheets and a method for producing the cold-rolled steel sheets, and more particularly to steel sheets having good bake-hardenability and press formability and a method for producing such steel sheets.
2. Description of the Prior Arts
In view of reducing vehicle weight, saving fuel consumption and increasing safety, more highly strengthened steel sheets for automobiles have been strongly demanded. Above all, high strength cold-rolled steel sheets for outer panels such as doors and hoods of automobiles, having tensile strength of 35 to 45 kg/mm.sup.2 and being good not only in bake-hardenability which is obtained by coating and baking process following press forming process but also in press formability. Continuous annealing has an advantage in giving such bake-hardenability to steel sheets. This is because the continuous annealing is so speedy in annealing and cooling, in comparison with box annealing that it can retain much of solute carbon in cold-rolled steel sheets. The continuous annealing has such a speed in the annealing and cooling process that steel sheets produced by the continuous annealing process are inferior, in press forming and deep drawing properties, to those produced by the box annealing process. For this reason, various methods of producing steel sheets which are good in press forming property, especially in deep drawing property have been studied. For example, a method for producing cold-rolled sheets having good deep drawing property is disclosed in a Japanese Patent Application Laid Open (KOKAI) No. 80726/84 as follows:
(a) using slabs consisting of: PA1 C : 0.010 wt. % and less; Si: 0.5 wt. % and less; PA1 Mn : 0.5 wt. % and less; N: 0.005 wt. % and less; PA1 Sol. Al: 0.05 to 0.20 wt. %; P: 0.03 to 0.15 wt. %; and the Rest : Fe and impurities inevitable;
(b) hot-rolling the slabs at finishing temperature of 650.degree. to 850.degree. C. and at coiling temperature of 560.degree. C. and less to produce hot-rolled steel strips; and
(c) cold-rolling the hot-rolled strips under the conventional terms to produce cold-rolled steel strips, followed by the process of continuous annealing the cold-rolled strips under the conventional terms.
This method, however, is disadvantageous in the following points:
(1) Since extra-low carbon steel is, in general, difficult in allowing over-aging treatment to effectively work, it is impossible to effectively control solute carbon in amount. Consequently, the steel sheets produced by this method have much of solute carbon and high bake-hardenability, while their antiaging property at room temperature is inferior to the anti-aging property of steel sheets heat-treated by box annealing process.
(2) Owing to addition of P in order to improve strength, abnormal coarse grains are easy to be formed in the continuous annealing process. Consequently, the steel sheets produced by this method have duplex structure, and are apt to cause drop of ductility and occurrence of a deficit of surface coarseness when they are press-formed.
(3) Owing to the low finishing temperature, growth of abnormal coarse grains is further encouraged.